The uxuAB operon is composed of two genes coding for enzymes involved in hexuronate degradation. This operon is negatively controlled by the uxuR and exuR regulatory gene products. Fusions that brought lac gene expression under the control of transcriptional and translational signals within the uxuR gene were used to study uxuR regulation. These fusions were formed on plasmid cloning vectors constructed by Casadaban et al. (J. Bacteriol. 143:971-980, 1980). The transcriptional direction of the uxuR gene was deduced from the restriction pattern and the phenotypic properties of the new plasmids. The gene is transcribed counterclockwise on the standard Escherichia coli map, as is the uxuAB operon. Introduction in trans of a compatible plasmid carrying a wild-type uxuR gene in the lac fusion plasmid containing strain resulted in a decrease of beta-galactosidase synthesis. It was concluded that expression of the uxuR gene itself is repressed by its own product. The two other types of regulation found in the uxuAB operon, i.e., induction by fructuronate and catabolite control, also apply to the uxuR gene, whereas repression by the exuR repressor does not seem to occur for the uxuR gene.